In recent years, there have been proposed and developed various hydraulically-operated vane rotor equipped variable valve timing control devices, capable of locking a vane rotor at an intermediate position between a maximum phase-advance position and a maximum phase-retard position by means of a lock mechanism during a starting period of an internal combustion engine. To unlock the lock mechanism, working fluid (hydraulic oil) in either a phase-advance chamber or a phase-retard chamber is used. When unlocking the lock mechanism by the use of working fluid (hydraulic oil) in either the phase-advance chamber or the phase-retard chamber, owing to alternating torque transmitted from a camshaft, the vane rotor tends to flutter, and thus hydraulic-pressure fluctuations in the phase-retard chamber and the phase-advance chamber occur. Owing to such hydraulic-pressure fluctuations, arising from alternating torque, there is a possibility that the locked state cannot be easily released.
To avoid this, Japanese Patent Provisional Publication No. 2000-170509 (hereinafter is referred to as “JP2000-170509”) teaches that an exclusive electrical control system (concretely, an electric-motor control system), only used for the lock mechanism, is provided separately from a control system for working-fluid supply-and-exhaust control for phase-advance chambers and phase-retard chambers.